Fiberglass reinforced plastics (FRP) are used in a wide variety of applications because of their excellent balance of physical properties, cost, and processibility. This has led FRPs to be of use in the automotive industry in applications as diverse as oil pans, heat shields, rocker covers, suspension parts, grill opening panels, and exterior body panels. FRPs are used in construction as well. Each of these applications takes advantage of some of the unique property balance of the unsaturated polyester or vinyl ester FRPs.
Unsaturated polyester FRPs have been used in exterior body panel applications due to their corrosion resistance, strength, and resistance to damage. The automotive industry has very stringent requirements for the surface appearance of these body panels. This desirable smooth surface is generally referred to as a "class A" surface. Unsaturated polyester resins typically shrink 5-8% on a volume basis when they are cured. In an FRP, this results in a very uneven surface because the glass fibers cause peaks and valleys when the resin shrinks around them. Special ingredients, called thermoplastic low profile additives, have been developed in order to help these materials meet the stringent surface smoothness requirements for a class A surface. These LPAs are typically thermoplastic polymers which compensate for curing shrinkage by creating extensive microvoids in the cured resin. Unsaturated polyester resins can now be formulated to meet or exceed the smoothness of metal parts which are also widely used in these applications.
In addition to LPAs, formulations contain large amounts of inorganic fillers such as calcium carbonate. These fillers contribute in two critical ways towards the surface smoothness of these compositions. First, the fillers dilute the resin mixture. Typically, there may be twice as much filler as resin on a weight basis in a formulation. This reduces the shrinkage of the overall composition simply because there is less material undergoing shrinkage. The second function of the filler is in aiding the microvoiding that LPAs induce. These stress concentration points seem to induce more efficient void formation.
In recent years, there has been added pressure on the automotive manufacturers to reduce the weight of cars in order to improve gas mileage. While FRPs have an advantage in this respect compared to competitive materials because of lower specific gravity, the fillers mentioned previously cause the part to be heavier than necessary. Most inorganic fillers have fairly high densities. Calcium carbonate, the most commonly used filler, has a density of about 2.71 g/cc, compared to a density of about 1.2 g/cc for the cured unsaturated polyester. A common FRP material used in body panel applications will have a density of about 1.9 g/cc. If this could be reduced by 10 to 20% while maintaining the other excellent properties of unsaturated polyester FRPs, a significant weight savings could be realized.
There have been many approaches to lowering the density of FRP materials. One approach has been to use lower density fillers in the formulations. For example, according to the Kirk-Othmer Encyclopedia of Chemical Technology 1980, wood and shell flour with specific gravity of 0.19 to 1.6 have been used. These have not been completely successful due to the lack of available low density fillers which are cheap, non-hygroscopic and have low coefficients of linear thermal expansion and small particle sizes. A second approach is to simply use less filler. As noted in U.S. Pat. No. 5,246,983, this approach has not been successful because removing filler causes the surface to degrade to the extent that it is no longer acceptable in appearance parts.
A more recent approach in U.S. Pat. No. 5,246,983 uses hollow glass spheres of very low density to compensate for the higher density fillers such as calcium carbonate. However, this approach is not without its problems. If the glass spheres are too large, a rough surface may result. Although U.S. Pat. No. 5,246,983 reveals a method for solving this problem by using small spheres, a more serious problem remains. These FRP parts must be primed and painted to allow the auto manufacturers to match colors and obtain acceptable quality surfaces. In nearly every painting operation, some defects occur, requiring the paint operators to sand and then refinish these blemishes. When an article which contains the glass bubbles is sanded, some of the bubbles are broken. When these areas are then refinished, these broken bubbles cause paint pops (more blemishes). This necessitates more sanding and refinishing, which simply compounds the problem.
An additional problem that faces molders of FRP parts is the cost of the tooling needed to mold acceptable pans. A tool typically is made of machined steel because of the high pressures needed (usually in the range of 800 to 1000 psi) (5.5-6.9 mega Pascals). If a molding compound could be developed which could be molded at lower pressures and still give acceptable surface and low density, lower cost tooling materials could be employed. Therefore, the problem remains as to an effective formulation and process for making lower density FRP parts which have acceptable surface and other properties including paintability, without the sacrifices of the current materials.
The literature generally recognizes the limited usefulness of clay as a filler. It is commonly listed among inorganic fillers useful in sheet molding compounds (SMC) a type of FRP. Clay is usually added as a secondary filler, useful for its rheological properties. It has not been recognized as a useful filler for enhancing surface quality in automotive appearance pans. Research has shown that mixtures of clay and calcium carbonate, for example, provide poor surface quality at low filler levels. In contrast to this, we have found that when clay is used as the sole filler at low levels (typically 10-120 parts based on total resin, in contrast to 180-200 parts calcium carbonate which are typically used) excellent surface quality can be maintained while giving lower density materials. In addition, we have found that these materials can be molded at pressures between 50 and 500 psi (0.34-3.4 mega Pascals) and still give good surface quality. Thus, this invention provides a material and process for producing FRP parts with good surfaces that have not been possible previously.